It is common in the machine tool industry to use superabrasive grinding wheels to shape and finish workpieces, and more specifically to grind inner and outer diameters of openings and bores, or to contour the surface of a workpiece. The term "grinding" will be used generally herein to describe any of the variety of processes for shaping and finishing parts, including polishing, working, lapping, grinding, contouring or otherwise finishing a workpiece surface. In almost all machine tool operations, the friction between the tool and workpiece generates tremendous amounts of heat energy (which can reach temperatures of about 2000.degree. F. or 1100.degree. C. and above) which if left uncontrolled, could severely damage (e.g., cracking or fracturing) the tool, thus reducing its tool life, making machine tool operations more dangerous and expensive, and reducing the quality and precision of the workmanship. In addition, heat generated friction can discolor the workpiece, and can damage or remove temper or heat treatment on the workpiece. It is commonly known in the industry that coolant can be introduced to the grinding area, such as by spraying, to reduce friction between the tool and workpiece by keeping a thin film of coolant fluid between the grinding wheel and workpiece, and also to help remove energy generated in machine tool operations.
Even though coolant fluid can be supplied to the grinding area, it is often difficult to insure that such fluid actually makes its way to the interstices between the tool and the workpiece, and fluid tends to quickly evaporate due to the high temperatures involved in grinding operation. Thus, large volumes of coolant fluid must generally be continuously supplied to the grinding area for the grinding wheel to operate effectively. This need to keep a thin continuous film of coolant fluid between the grinding wheel and workpiece becomes even more problematic in operations where coolant fluids cannot be introduced in close proximity to the grinding areas while the grinding wheel is engaged with a workpiece due to, for example, the depth of the grinding action in the workpiece.
During use, the grinding surfaces of tools such as grinders can become loaded (e.g., plated or plasticized) with particles from the workpiece, which in turn, reduces the effectiveness of the tool through deteriorating grinding ability, scratching of a workpiece, and even clogging of conventional coolant fluid supply openings. It is obviously preferred that the potential for this undesired loading of particles be reduced, and that any loaded particles be removed from the grinding wheel as quickly as possible. Typically, nozzle arrangements, such as an external cleaning jets, are provided independent of the tool, for injecting coolant fluid at increased velocities toward the grinding surface to wash away particles, to remove plasticized particles already formed on the work surface, and to cool the grinding wheel and workpiece. As mentioned before, it is often very difficult to insure that the fluid sprayed in this way actually reaches the most critical areas of the tool/workpiece interface.
Previously, attempts to address these two simultaneous requirements of cooling the grinding wheel and workpiece and cleaning the grinding wheel have tended to also reduce the flexibility and utility of a machine tool. For example, deep cuts, such as undertaken in creep feed grinding, are difficult to make as coolant delivery to the grinding area is generally limited by the volume of coolant fluid which can be supplied by spraying techniques to the grinding area, and as a result, plasticizing of particles on the grinding wheel as well as heat generated by friction often reduces a tool's effectiveness.
One attempt to more effectively cool tools and hard abrasive workpieces is disclosed in U.S. Pat. No. 3,233,369 to Highberg, where coolant fluid is directed from an external source into an enclosed vertical passageway of a spindle. Fluid is then discharged onto the work surface through orifices adjacent to the inner cutting edge and/or through the center of the tool. Highberg relies primarily on gravity and centrifugal forces (as opposed to high pressure) to deliver coolant fluid through the orifices to the cutting edge. There are, at least, several outstanding shortcomings in the contemplated Highberg system. First, the tool would appear to be primarily limited to a vertical orientation because of the gravitational fluid delivery mechanism, thereby limiting its application and adaptability. Secondly, it is believed that coolant fluid will generally not be delivered in such an arrangement at a sufficient velocity to clean the grinding edges of a superabrasive wheel while in use. As a result, Highberg would not provide a flexible superabrasive grinding system that can provide unrestricted tool paths. Highberg also requires the use of one or more external nozzle jets to provide a fluid spray and remove loaded particles from the superabrasive grinding wheel. Because the use of external jets of this type generally requires alignment procedures to apply coolant at the appropriate angle and location, adjustments and other timely reconfiguration procedures of the coolant supply system would be required when the tool configuration and/or tool path is varied.
Another attempt to improve grinding wheels is described in U.S. Pat. No. 3,244,739, to Brutvan et al. In this device, coolant fluid is supplied from the center of the wheel to the outer grinding edge through guide channels and onto the workpiece via centrifugal force to the point of contact with the grinding wheel. By having an integral, and allegedly even and continuous flow of coolant fluid over the cutting edge, small workpiece particles are to be washed away, and particle build up is theoretically avoided. Brutvan, et al. contemplates that overheating is addressed by the coolant flow equalizing the temperature over the entire workpiece which presumably allows for deeper and heavier cuts to be made. Although Brutvan is designed to wash away particles, its arrangement does not appear to supply coolant fluid at sufficient velocity (i.e., under high enough pressure) to prevent or remove loaded particles on a superabrasive grinding wheel. As mentioned above, the operational speeds and higher temperatures of superabrasive grinding operations tend to overwhelm the centrifugal force application of coolant fluid. Additionally, the machine tool in Brutvan appears to be more suited for a dedicated operation or fixed to a machine, and would not appear to be easily adaptable in a tool holder used with an automatic or quick tool changing system. Again, external nozzle jets for additional coolant delivery would most likely have to be used with the tool shown in Brutvan et al. to help reduce the possibility of plasticizing or plating of particles, and to remove the already loaded particles.
Other attempts to deliver coolant fluid to the grinding area have used air or other pneumatic carriers. As with externally applied liquid coolants, when pneumatic carriers are used, however, turbulence can hinder the grinding operations and often fluid cannot infiltrate into the actual grinding areas.
As can be seen, currently available grinding tools have a number of shortcomings which greatly reduce the ability to use these tools with automatic tool changing systems. Moreover, superabrasive grinding wheels generally operate at increased rotational speeds which result in increased temperatures being generated and increased pressure being exerted on the workpiece by the wheel. A wheel operating under these conditions generally requires additional external coolant fluid supplies or jets to reduce or remove loaded particles from the grinding wheel or to cool the workpiece and grinding wheel. The industry currently lacks a superabrasive grinding tool configured to allow for use of the tool in a wide range of operations (i.e., grinding the inner or outer diameter of a workpiece or face grinding) utilizing a variety of tool paths and which can be used in a quick change machine tool center while also allowing for efficient and enhanced deep precision grinding.